Page 223 - A Practical Guide from Design Planning to Manufacturing

196 Chapter Six

PS
PS 1 0
F-F
PS Tk NS 1 PS 1
0
Tk D Q Prd
PS Tk Clk
1

F-F
PS Tk NS 0 PS 0
0
PS 0 D Q
PS PS 0 Clk
1
Branch
Clock
Figure 6-25 Branch prediction implementation.

Conclusion
At its heart, logic design is this process of taking an idea for a microar-
chitectural feature and changing it into an implementation of logic
gates ready for the next step of circuit design. Logic design starts with
a microarchitectural specification, which may be no more than a text
document. It transforms this idea for a processor into HDL models that
can be validated and implemented. Pre-silicon validation is critical to
make sure operation of the first prototype chips is as close to the orig-
inal specification as possible, but inevitably some bugs are found only
in post-silicon validation. Making pre-silicon validation faster and
more thorough is one of the most important needs of future processor
designs.
Automation tools allow logic design to be the last step before manu-
facturing for some designs or at least parts of some designs. These auto-
mated flows allow design time to be reduced at some sacrifice in area
and speed. It is possible to support many different levels of automation
to allow the appropriate level of hand control to be chosen separately
for each part of the design. When optimizing combinational logic by
hand, Karnaugh maps are commonly used, and sequential circuits are
designed in similar fashion by treating the current state of the circuit
as just another set of inputs.
Logic design does not consider all the details of the real transistors
that will be used to make the logic circuit, but in converting a specifi-
cation into a model that can be simulated and tested for correctness, logic
design makes the critical leap from idea to implementation.